Should the surround speakers be made by the same company that makes the fronts?

In an ideal setup, YES! At the very least your center and your fronts should match perfectly. IN MY OPINION if you have the same quality level speaker on the rear, but a different brand, it may sound OK and it may not depends on the speaks and your ears.

Of any of your speakers this is the least important. But with some of the new DTS and 5.1 music engineers are starting to use the rears as fullrange speakers. For these recordings it becomes more important to match with your fronts.

Hope this helps!